The UN's nuclear watchdog has issued a rare condemnation to Iran over its secretive uranium processing operations.

Twenty-five nations backed a resolution that demands Tehran immediately freeze construction of its newly revealed nuclear facility and heed Security Council resolutions calling on it to stop uranium enrichment.

Iran remained defiant, with its chief representative to the International Atomic Energy Agency declaring that his country would resist “pressure, resolutions, sanction(s) and threat of military attack”.

The resolution — and the resulting vote of the IAEA's 35-nation decision-making board — were significant on several counts.

The resolution was endorsed by six world powers — the US, Russia, China, Britain, France and Germany — reflecting a rare measure of unity on Iran. Moscow and Beijing have acted as a traditional brake on efforts to punish Iran for its nuclear defiance, either preventing new UN Security Council sanctions or watering down their potency.

Their backing for the document at the Vienna meeting thus reflected broad international disenchantment with Tehran.

The resolution sends a strong signal to Iran that its actions and intentions remain a matter of grave international concern.

The backing of Moscow and Beijing also appeared to signal possible support for any new Western push for a fourth set of UN sanctions, should Tehran continue shunning international overtures meant to reach agreements that reduce concerns about its nuclear ambitions.

British Foreign Secretary David Miliband suggested sanctions could be a next consideration in a statement saying the six powers remained committed to their “dual track” policy -— a term alluding to attempts to engage Iran diplomatically but to turn to sanctions should the first track fail.

“We are waiting for Iran to respond meaningfully,” he said. “But if it is clear that Iran has chosen not to do so, we will have no alternative but to consider further pressure on Iran, in line with the dual track policy we have been pursuing.”

Iran argues that attacks on its nuclear programme are an assault on the rights of developing nations to create their own peaceful nuclear energy network.

But the IAEA resolution criticised Iran for defying a UN Security Council ban on uranium enrichment — the source of both nuclear fuel and the fissile core of warheads.